Many robots are adopted in, for example, a factory for labor savings. Most of the robots are operated while giving priority to, for example, speed, accuracy and efficiency.
In recent years, robots have been used to take care of people. In such a field, robots operate in the neighborhood of people, and considerations from another aspect, such as prevention of danger from being posed on people, need to be given.
As regards, for example, a robot arm, a linearly-extensible and retractable arm mechanism adopting, instead of a rotational joint, a linear motion joint for making an arm extend and retract, which allows the arm to linearly extend when the arm extends, and requires relatively less space when the arm retracts, has been conceived and been in the actual use.
As the linearly-extensible and retractable arm mechanism, a structure including two structure assemblies which form a rigid arm part when they are placed one upon another and extended by a drive mechanism, but are bent and stored in an arm support when they are separated by an inverse rotation of the drive mechanism, has been known.
However, the two structures of the linearly-extensible and retractable arm mechanism placed one upon another may be separated in their tracks, and do not form a sufficiently-secure rigid body.